This invention relates in general to photolithography and, more particularly, to a photomask and method for coating a back-side of the same with an anti-reflective material.
Photolithography systems use an illumination source and a photomask assembly to project an image on to the surface of a semiconductor wafer. A photomask assembly typically includes a photomask with a patterned absorber layer formed on the front-side of a transparent substrate and a pellicle mounted on the photomask to protect the photomask from contaminants in the photolithography system. Complex geometries, such as integrated circuits (ICs), are formed on the surface by passing radiant energy from the illumination source through apertures in the patterned layer of the photomask and focusing the image on a layer of resist that is coated on the wafer.
The radiant energy directed at a photomask assembly will either pass through the photomask (sometimes referred to as transmission), be absorbed within the photomask, or be reflected off of the photomask. For photomasks in which absorption remains substantially constant, any decrease in reflection results in an increase in transmission through the photomask. Traditionally, an anti-reflective coating may be added to the front-side of the photomask over the patterned absorber layer to reduce the amount of reflection from the surface.
In other embodiments, conventional photomasks have included two layers of anti-reflective coating. A first anti-reflective layer is formed directly on the front side of the substrate and a second anti-reflective layer is formed on the patterned absorber layer. In such a photomask, the absorber layer is located between the two anti-reflective layers. These techniques may reduce reflection, however, transmission loss still occurs due to reflection from other portions of the photomask. Additionally, the anti-reflective layer formed on the front-side of the substrate may introduce stresses on the substrate causing the photomask to bend or warp. Such bending or warping may distort the image when projected onto a target wafer, thereby leading to defects and errors in the resulting wafers.
Additionally, unwanted reflection off of a photomask may exacerbate so called iso-dense bias effects. Iso-dense bias is a common optical proximity effect that creates a difference between the printed dimensions of an isolated feature and a dense feature having the same design dimensions. In dense areas, more light is reflected from dense areas of an absorber layer. This causes more light to be reflected onto the wafer surface. Such unwanted reflected light increases the likelihood of critical dimension and pattern registration errors.
In accordance with the teachings of the present invention, disadvantages and problems associated with the reflection of radiant energy in a photomask assembly have been substantially reduced or eliminated. In a particular embodiment, the back side of the photomask includes an anti-reflective layer that substantially reduces reflection from the back side of the substrate. The inclusion of the anti-reflective layer on the back-side of the substrate improves the transmission of light through the photomask and decreases unwanted reflections.
The physical properties of transmission (T), reflection (R), and absorption (A) are related through the following formula:
T+R+A=1
As the light goes through a medium, by varying any one or two of these three properties, the other will be affected by use of this simple relationship. The current invention of backside anti-reflective coating is based on this fact as explained below.
In one aspect the present invention includes a photomask having a transparent substrate with a patterned absorber layer formed thereon. The substrate has both a front-side surface and a back-side surface. Additionally, an anti-reflective layer is deposited on the back-side of the substrate. More particularly, the anti-reflective coating may have a refractive index between 1.4 and 1.8.
In another aspect of the present invention, a method for reducing reflection in a photolithography system is described that includes providing a transparent substrate. The method also includes coating the backside surface of the substrate with an anti-reflective coating. In a particular embodiment, the anti-reflective coating may be applied to the back-side surface of the substrate using an evaporation technique, a sputtering technique or a spin-coating technique.
Important technical advantages of certain embodiments of the present invention include a photomask that improves the exposure uniformity across a wafer and reduces iso-dense bias effects on the wafer. In the present invention, light reflected from the absorber layer is not reflected from the back side of the substrate and down to the wafer surface. In this manner, the anti-reflective material reduces the iso-dense bias effects on the wafer that are caused by the unwanted reflected light at the surface of the wafer.
Another important technical advantage of the present invention is that the back-side anti-reflective layer improves the photolithography system throughput by decreasing overall reflectivity and thereby increasing the transmission of light to a wafer surface. Mathematically this comes from the above formula as the incident light goes through the substrate material the current invention will increase R, since T=1-A-R, as R increases and A remains near constant transmission T has to increase through the substrate thus improving the throughput of the optical system.
A further important technical advantage of the present invention includes a photomask that decreases critical dimension and pattern registration errors on a wafer surface. On a conventional photomask, critical dimension and pattern registration errors may occur on the wafer surface because stresses in the photomask have caused the photomask to bend or warp. These stresses effect the flatness of the photomask and cause printing errors. In the present invention, the addition of an anti-reflective layer on the back-side of the photomask may preferably introduce intrinsic stresses from the back side of the substrate. These stresses may balance or counteract the stresses from the front side of the substrate, and thus produce a photomask with increased flatness characteristics. Other technical advantages will be readily apparent to one skilled in the art from the following figures and descriptions.